Method of producing corrosion resistant coatings of uniform color and appearnce on asuccession of aluminum pieces



United States Patent ()fiice Z ,8 l 4 ,5 77 Patented Nov. 26, 1957METHOD OF PRODUCING CORROSION RESIST- ANT COATINGS OF UNIFORM COLOR ANDAP- PEARANCE ON A SUCCESSION OF ALUMINUM PIECES Nelson J. Newhard, Jr.,Oreland, Pa., assignor to American Chemical Paint Company, Ambler, Pa.,a corporation of Delaware No Drawing. Application May 23, 1956, SerialNo. 586,646

1 Claim. (Cl. 1486.16)

This invention relates to the art of applying a chemically bondedcoating to the surface of aluminum or alloys thereof in which aluminumis the principal ingredieat, but before stating the objects of theinvention, it is desired to refer briefly to certain coating procedureswhich are now familiar to those skilled in the art.

It is well known that acidic solutions whose principal and essentialcoating producing ingredients are fluorides, hexavalent chromium andphosphates and/or arsenates are capable of producing decorative as wellas corrosion resistant coatings on aluminum and typical examples of suchprocesses are described in U. S. Patents 2,438,877; 2,471,909;2,472,864; and 2,678,291. Examination of these patents will show thatthe solutions used in the coating procedures, in general, have a pH ofbetween 1.6 and 2.2 and that the chemical constituents thereof arepresent in amounts corresponding to the following: Fluoride 0.6 to 12.5grams per liter; hexavalent chromium expressed as CrO 3 to 60 grams perliter; and phosphate (P and/or pentavalent arsenate (As O 2 to 285 gramsper liter. Furthermore, all of these patents indicate that the fluoridecontent of the bath must be maintained within a certain ratio to thehexavalent chromium content of the bath.

The solutions used in the methods just referred to are very importantcommercially and for quite a number of years past there has been agrowing demand for aluminum bearing such coatings because of theirdecorative and protective values. However, during continued use of thesolutions or baths employed with such processes, the color imparted to asuccession of aluminum articles when processed through the same bathtends to vary. Obviously this is a disadvantage especially where thearticles being processed are wall panels or sheets because of thedesirability of maintaining the appearance of walls substantiallyuniform in color as Well as in texture and corrosion resistance.

In prior practice it has been customary to maintain certain ratios amongthe chemical constituents of the solutions used such as thefiuoridezchromate ratio, etc., and this practice has been primarilyconcerned with the maintenance of uniformly corrosion resistantcoatings. However, as indicated, such practices have been found to beinadequate insofar as maintenance of uniformity in color is concerned.This inadequacy presents a particularly troublesome problem in thearchitectural field where substantial uniformity of appearance isimportant.

With the foregoing in mind, the principal object of the presentinvention will be better understood and more fully appreciated; and thisobject is the provision of a method of coating a succession of aluminumpieces in a substantially continuous process involving use of a properlyreplenished solution by means of which it is possible to maintain oneach piece substantial uniformity in appearance and color as well as incorrosion resistance.

As a succession of pieces of aluminum are coated, it is, of course, wellknown that aluminum as well as alloying elements in alloys of aluminum,are dissolved in the bath and I have found that this is largelyresponsible for the marked change in the appearance and color of thework being processed. Additional experience has now shown that thechange in color is due in a very large degree to the manner in which thedissolved metal afiects the fluoride content of the bath. Ordinarychemical analysis may show that a used bath has the same totalfluoridezchromate ratio as did the original bath but, despite this, thecolor which it imparts to a coated surface is markedly different fromthe color produced by the same bath when new or fresh. Furthermore,commercial formulations of the kind in question quite often have, as asource of fluoride, a complex fluoride, for the reasons mentioned in thepatents referred to. Certain advantages accrue from the use of suchcomplex fluorides but their presence in the bath further complicates thebath chemistry insofar as the fluoridezchromate ratio is concerned.

The present invention is based upon the discovery that, if themaintenance of a set and definite fiuoride:chromate ratio is dispensedwith and if, for lack of a better expression what may be termed activefluoride, is kept substantially constant, regardless of the actualfluoride:chromate ratio, the bath will produce corrosion resistant anddecorative coatings of uniform color and appearance on a succession ofaluminum surfaces. Naturally, the concentration of hexavalent chromiumand phosphate or arsenate should be maintained at conventionalconcentrations by conventional means.

It should be noted that the term active fluoride is used herein to meanthe ability of the coating bath to etch a piece of lime soda glass asmeasured by weight loss.

In order to carry out the present invention all that is necessary is tomake up a coating bath in accordance with the knowledge of the art toproduce the desired color on aluminum or aluminum alloys and thenmeasure its fluoride activity. This may be done with ordinary microscopeslides of lime soda glass. The measurement is preferably accomplished byheating a convenient volume of the bath to approximately 115 F. and thenimmersing therein a 4 inch by 3% inch microscope slide for approximatelyone-half hour. By taking the weight before and after the immersion, theweight loss is readily determined. As work is processed through thebath, sufficient fluoride is added from time to time as may be necessaryto maintain substantially constant the original fluoride activity.

By way of example, but without limitation as to the specific ingredientsor concentrations, work may be processed through a bath consistingprincipally and essentially of the ingredients given in the followingformula:

Water, to make 1 liter.

Such a bath is capable of producing a soft green color when used in adip process at F. with a treating time of approximately 1 /2 minutes. Inthis particular case the fluoride activity was measured by immersing alime soda microscope slide in a part of the bath for /2 hour at atemperature of F. The microscope slide was approximately 4 x 3% inchesin size. The fluoride activity caused a loss of 39.5 mgs. of glass.

To this bath .325 gram of aluminum was added in the form of aluminumphosphate thus simulating the composition of a used bath having the samefluoridemhromate ratio as the bath when new. An aluminum panel wasprocessed with a portion of this bath in a similar fashion to that givenabove. The color imparted to the coated panel was of a dull greyishgreen color and quite different in appearance from the color of thepanel coated with a portion of the new bath. The fluoride activity ofthis bath was measured in the same manner as above and found to be downto 17.2 mg. less in weight. An addition of 1.4 grams of sodiumbifluoride to the bath brought the fluoride activity back to the levelof the original bath. Panels processed in this refurbished bath had thesame color as the first panel.

I claim:

In the art of coacting a succession of aluminum pieces by subjecting thesurface thereof to the action of an aqueous acid coating solutionwherein the coating producing ingredients consist essentially offluoride ions, chromate ions and ions from the class consisting ofphospates and arsenates, the method of replenishing the solution whichincludes adding thereto compounds containing chromate ions and ions fromthe class of phosphates and arsenates sufficient to maintain therequired concentrations thereof plus fluoride in suflicient quantity tomaintain the fluoride activity of the solution substantially constant asmeasured by determining the weight loss of a standard piece and type ofglass when immersed in the solution for a given period of time at asubstantially constant temperature.

References Cited in the file of this patent UNITED STATES PATENTSSpruance Mar. 30, 1948 Metler June 1, 1954 OTHER REFERENCES

